Rechargeable electrical storage battery with zinc anode and aqueous alkaline electrolyte

ABSTRACT

A battery system having a zinc containing anode in an alkaline electrolyte produced in a manner to substantially avoid dendritic growth and anode shape change with additives added to the electrolyte and/or anode to assist therein.

This is a division of application Ser. No. 100,845, filed Dec. 6, 1979,now U.S. Pat. No. 4,307,164, as a division of Ser. No. 927,927 filedJuly 25, 1978, now U.S. Pat. No. 4,207,391.

BACKGROUND

This invention is directed to an improved use of zinc in rechargeableelectrical storage batteries incorporating an aqueous alkalineelectrolyte and wherein the zinc is used as the negative electrode, i.e.anode. Typical positive electrodes (cathodes) used in conjunction withthese zinc anodes include nickel oxide, AgO, Ag₂ O, MnO₂, HgO, PbO₂ andthe like.

The use of zinc electrodes in rechargeable batteries has long beenrecognized as a means of obtaining high energy density and high batteryvoltage compared to less electronegative anode materials such ascadmium, iron or lead. Zinc also has the advantage of being relativelylight in weight, relatively abundant and relatively low in cost.

As is well known to those involved in the field, zinc electrodes whereused in an alkaline electrolyte battery, have a habit of formingdendrites or "trees" during repeated charge/discharge cycling. Thissignificantly shortens the useful cell life. If left unchecked, thesedendrites or tree-like crystalline zinc growths will quickly bridgeacross to the positive electrode, usually causing battery shorting in avery few use cycles.

Associated with the dendrite problem at the anode is the so-called shapechange phenomenon. This is a shifting of the zinc on the anode currentcollector that can cause a serious reduction in battery efficiency. Thedendrite and shape change effects are considered by many to be closelyinterrelated, and are thought to be linked to the solubility of zinc inthe alkaline electrolyte solution.

Cell shorting by dendrite growth can be slowed down by the inclusion ofmicroporous membranes as part of the separator system used between theelectrodes. Other techniques such as intermediate electrodes thatoxidize the dendrites on contact and electrolyte additives that act toprevent zinc from going into solution have been disclosed. Speciallyshaped anodes have been found helpful in some cell configurations forreducing shape change. While cycle life can sometimes be increased bythese and other methods, this is invariably accomplished at the expenseof some other factor. Perhaps primary among these are increased internalcell resistance, increased battery size and weight and increased batterycost. Special electrical charging systems have been reported to slowdendrite growth or change their crystalline form.

None of the approaches reported to date, when used singly or incombination with each other, seems to have adequately solved the zincanode problem for long cycle life rechargeable storage batteries. Aviable solution to this problem is currently being sought by manyinvestigators. The closest prior art patents of which applicant is awareare as follows:

U.S. Pat. No. 3,617,384,

U.S. Pat. No. 3,672,996,

U.S. Pat. No. 3,785,868,

U.S. Pat. No. 3,970,472

SUMMARY OF THE INVENTION

The present invention is based on what is believed to be a new principlefor preventing or greatly minimizing dendrite growth and shape change atthe zinc anode of aqueous alkaline electrolyte rechargeable cells. Alltesting to date has been carried out in cells without the use ofmicroporous membranes of any kind. Separation has been entirelyaccomplished by means of highly porous non-woven nylon or viscosematerial.

The new approach consists of a special anode configuration where thezinc is present in a microcrystalline or colloidal state incorporated ina conductive oxidized zinc matrix. This is accomplished byelectrochemically converting a finely divided metallic zinc anode to ahighly oxidized form in the presence of an alkaline electrolyte.Subsequent charging of the anode will cause the formation of therequired amount of interstitial zinc. These electrochemical oxidationand reduction processes may be conveniently carried out within theactual cell in which the anode is to be used.

The invention also includes the use of selected inorganic additionsincorporated in the electrolyte and/or anodes. These additives consistof certain manganese, tin and related compounds. When employed with theelectrochemically converted anodes, their use has resulted in dendritefree cells with minimal shape change. Comparison cells, using moreconventional zinc anodes and electrolyte systems, have shown rapidfailure due to severe dendrite growth and associated shape change in thesame number of charge/discharge cycles using the same test conditions.

The fact that microporous membranes are not required in the practice ofthis invention has been found to result in low internal cell resistance.This results in a highly efficient charge to discharge relationship, andincreased energy density.

DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a test cell according to the invention with FIG. 1being a cross sectional view along line 1--1 of FIG. 2; and,

FIG. 3 is an exploded view of the test cell of FIGS. 1 and 2 less theplastic envelope.

The special anode of this invention consists of an electrochemicallyoxidized anode that initially was substantially composed of finelydivided metallic zinc particles bonded to a suitable current collector.The metallic zinc employed in the initial anode may be zinc dust, zincpowder or other forms of finely divided zinc. A nominal 6 micron zincdust has been employed in most of the testing described in thisdisclosure, but zinc powders ranging in size from 5 microns to -325mesh, and even -100 mesh, have been used successfully.

In constructing the anode, the zinc dust or powder has typically beenmade into a thick slurry using a binder consisting of a premixed waterand methylcellulose mixture. Mercuric oxide or some other mercurycompound is also normally included in the mixture to increase thehydrogen overvoltage of the zinc according to well-known practice. Abasic zinc slurry formulation can be seen by referring to anode type Iin Table 1.

The zinc slurry formulations shown in this table have been designed forapplication to the current collector by means of a so-called "pastingjig". In use, a suitable current collector such as a metal foil, metalscreen or expanded metal sheet is placed in the bottom of anappropriately shaped recessed area in the jig. The slurry is thentroweled into and across the cavity containing the collector, providinga uniform thickness zinc anode.

When using expanded metal or open mesh screen as the current collector,a piece of absorbent material, such as high rag content paper, is firstplaced in the recess of the "pasting jig" to prevent the slurry frompenetrating the collector and sticking to the jig surface. After theslurry has been applied to the collector, the still wet anode is removedfrom the jig and allowed to dry. The bond between zinc and currentcollector will be sufficiently strong at this point for normal handlingprocedures. Any flashing, or excess material at the edges of the currentcollector or on the current collector tab can be easily removed in thisdry condition.

While a methylcellulose-water mixture has been used as the binder in theformulations of Table 1, other binders appear to be useful such as avariety of sodium carboxy methylcellulose compositions, polyvinylalcohol, Teflon and even small amounts of KOH. Again, while the solventhas normally been water, other solvents may be employed with or withoutwater, depending on the specific binders involved, the drying timesdesired or the anode additivies used.

Other methods of fabricating the zinc anodes have been successfully usedsuch as dipping the current collector into a suitably thinned slurry, orpainting or flowing the slurry onto the collector. Methods of this typemay well be preferable for production quantities, but the pastingtechnique was found to be the most expeditious when preparing smallnumbers of anodes from a limited amount of slurry.

Anode construction of this type is not new, having previously been usedby others. For example, Devitt in U.S. Pat. No. 3,785,868 describesanodes of this basic form. Numerous other methods for preparing zincanodes of usable form have also been described in prior literature.

Current collectors used in this development have included nickel andcopper as well as tin, silver and gold plated nickel. All of these havegiven excellent results with repeated cycling. The nickel would probablybe avoided in commercial cells because of cost and, in the unplatedcondition, also because of its known property of reducing the hydrogenovervoltage of zinc. This could reduce battery open circuit life on longterm standing. The least expensive current collector for production usewould most likely be copper or tin plated iron or steel.

The metallic zinc anodes of the type just described can now be used toform the novel anode of this invention. This involves a specialelectrochemical anode oxidation step with the metallic zinc anodeimmersed in an aqueous alkaline electrolyte solution.

                  TABLE 1                                                         ______________________________________                                        SELECTED ZINC ANODE SLURRY COMPOSITIONS                                       SUITABLE FOR USE IN PASTING JIG                                                                                   PER-                                      FORMU-                     A-       CENT.sup.(a)                              LATION                     MOUNT    ADDI-                                     #       COMPOSITION.sup.(h)                                                                              (grams)  TIVE                                      ______________________________________                                        I       Zinc dust.sup.(b)  24.38    97.5                                              Hgo.sup.(d)        0.63     2.5                                               Methylcellulose - H.sub.2 O binder.sup.(c)                                                       4.60                                               AS      Zinc dust          23.90    95.5                                              Hgo                0.63     2.5                                               Cu - 400 mesh      0.50     2.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               BA      Zinc dust          23.90    95.5                                              Hgo                0.63     2.5                                               Pb, -325 mesh      0.50     2.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               BB      Zinc dust          22.90    91.5                                              Hgo                0.63     2.5                                               Pb, -325 mesh      1.50     6.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               EF      Zinc dust          23.40    93.5                                              Hgo                0.63     2.5                                               SnO.sub.2.sup.(e)  1.00     4.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               EG      Zinc dust          23.90    95.5                                              Hgo                0.63     2.5                                               SnO.sub.2          0.50     2.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               EK      Zinc dust          24.15    96.5                                              Hgo                0.63     2.5                                               Sn, -325 mesh      0.25     1.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               EJ      Zinc dust          23.90    95.5                                              Hgo                0.63     2.5                                               Sn, -325 mesh      0.50     2.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               EW      Zinc dust          23.90    95.5                                              Hgo                0.63     2.5                                               ZnSnO.sub.3.4H.sub.2 O.sup.(f)                                                                   0.50     2.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               EX      Zinc dust          23.65    94.5                                              Hgo                0.63     2.5                                               ZnSnO.sub.3.4H.sub.2 O                                                                           0.75     3.0                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               FA      Zinc dust          24.4     95.5                                              Hgo                0.63     2.5                                               K.sub.2 SnO.sub.3.3H.sub.2 O.sup.(g)                                                             0.5      2.0                                               Methylcellulose - H.sub.2 O binder                                                               4.6                                                FB      Zinc dust          24.40    93.7                                              Hgo                0.63     2.4                                               ZnSnO.sub.3.4H.sub.2 O                                                                           0.50     1.9                                               K.sub.2 SnO.sub.3.3H.sub.2 O                                                                     0.50     1.9                                               Methylcellulose - H.sub.2 O binder                                                               4.60                                               ______________________________________                                         NOTES                                                                         .sup.(a) Percentages do not include methylcellulose  H.sub.2 O binder as      an insignificant weight remains after the water evaporation                   .sup.(b) Asarco type 111 zinc dust, -6 micron particle size, manufactured     by Asarco Corporation, So. Plainfield, N.J.                                   .sup.(c) METHOCEL, type A4M methylcellulose, 4000 cps viscosity,              manufactured by The Dow Chemical Company, Midland, Michigan                   .sup.(d) Mercuric oxide, yellow form, fine powder                             .sup.(e) Stannic oxide, fine powder                                           .sup.(f) Zinc stannate, chemically precipitated from zinc nitrate,            potassium stannate reaction, washed and dried                                 .sup.(g) 95% potassium stannate                                               .sup.(h) The slurry composition ingredients have been extremely well mixe     to assure uniform particulate dispersion. The procedure involves premixin     the HgO powder with the methylcellulose  H.sub.2 O gelled binder, then        adding and mixing any of the special anode additives. The final step is t     add the zinc dust. Thorough mixing is done at each of these steps.       

During this process, a high degree of oxidation of the zinc occurs andthe anode undergoes a considerable expansion. Electrochemicallyconverted anodes of this type have been found to provide greatly reduceddendrite growth and shape change as compared to cells using moreconventional zinc anodes.

Assuming a nickel-zinc cell as an example, an expedient manner ofcarrying out the initial electrochemical oxidation step is simply tostart with a completely assembled cell. The cell would include themetallic zinc anode, separator and cathode. An electrolyte, such as30%-40% KOH solution saturated or nearly saturated with zincate is thenadded to the cell. If the nickel cathode has been installed in itsdischarged condition, the electrochemical oxidation of the anode can nowbe accomplished by simply shorting the electrode terminals. Hydrogen gaswill start to evolve almost immediately from the nickel cathode. Thiswill continue until the zinc at the anode reaches a highly oxidizedstate.

With the shorting method just described, it has been found that themajority of the nickel-zinc cells discussed in this application willhave reached a highly oxidized anode condition in a period of 16 to 24hours. At this point, it will be observed that the gassing in the cellswill have essentially stopped. Microscopic examination shows an expandedand very white anode material which includes electrolyte. No visualevidence of the original zinc can be found, indicating a high degree ofconversion to the oxidized form.

Chemical analysis of such cell anodes have confirmed these visualobservations. For example, typical anodes removed from a cell after a 22hour shorting period were found to contain about 96% in an oxidized zincform with only about 4% remaining as zinc.

The anode can also be converted to a suitably oxidized state overshorter or longer time periods by using resistive loads, chargedcathodes, constant current discharging and the like. Care should betaken, however, not to overheat the electrodes as this can cause warpingof the cathode or non-uniformity at the anode.

The time required for the gassing to reach a minimal state during thedischarging cycle will depend on the amount of zinc used in the initialanode, on the discharge rate and also on the temperature. Because ofthis latter variable and for consistency of test results, an ambienttemperature of about 80° F. has been used for the special anodeprocessing in most of the examples in this disclosure.

The final anode preparation step occurs during the subsequent and normalelectrical charge cycle when a portion of the electrochemically oxidizedanode material is converted to extremely finely divided zinc. Thepresence of the zinc can now be observed as a blue coloration of theanode, indicating highly dispersed zinc in the oxidized zinc lattice.The zinc cannot be seen in a visible aggregate form under normalmicroscopic observation and must be assumed to be microcrystalline orcolloidal in nature. Chemical activity and an electrical output from thecell has confirmed its presence.

The reasons for the improved performance of anodes that have undergonethis high degree of initial electrochemical oxidation prior to normalcell cycling is not understood with any degree of certainty. Ourconclusions, however, based both on observations and supposition, can besummarized as follows:

(1) The anode has a high degree of electrical conductivity, probably dueto the inclusion of KOH from the electrolyte closely associated orchemically tied up with the oxidized zinc structure. This is evident bythe excellent electrical characteristics of such cells;

(2) Metallic zinc present in the highly reduced anode structure duringor at the conclusion of a charge cycle, is in a uniformly distributedand finely divided form. This condition provides a very low currentdensity at the individual zinc particles because of the extremely largesurface area presented. Prior work by others has invariably shown thatzinc dendrite reduction can be equated with increased surface area andcurrent density reduction; and,

(3) The shift of the active zinc material from a reduced to a moreoxidized form and back again to a more reduced form during repeated cellcycling can most easily take place within the highly conductive oxidelattice structure of the anode. As a result there is believed to belittle tendency for a mass transfer to occur between the cellelectrolyte solution and the anode. Plating of zinc onto the anode fromthe zincate in the electrolyte of cells containing zinc anodes isthought by many investigators to be a major cause of dendrite growth andassociated shape change problems.

While a highly oxidized anode condition prior to the initial chargecycle has been found to provide maximum dendrite prevention under severecell operating conditions, even incompletely oxidized anodes have oftenshown marked improvement over comparison cells with no preliminaryoxidation step.

The electrode separator materials used in the course of this developmenthave included non-woven nylon and in earlier stages a similarlystructured viscose material. Specifically, these have included types2505-k4 and 2503-k4 nylon manufactured by the Pellon Corporation ofLowell, Mass. The cellulose separator material was manufactured byChicopee Manufacturing Company of Mill Town, N.J. as their type S-950-CFCode 3674. The nylon separators are about 0.023 cm in thickness and thelatter about 0.018 cm in thickness.

In most instances, three individual separators have been used betweenthe zinc anode and the cathode. On cell disassembly, the multipleseparators afford a convenient gauge for determining dendritepenetration. These separators have a high degree of porosity allowingthe free flow of electrolyte between electrodes.

The electrolytes used in the test cells involved in this applicationhave all been based on a potassium hydroxide system, both with andwithout zincate additive. Compositions of these basic electrolytes havebeen listed for reference purposes in Table 2. Sodium hydroxide, lithiumhydroxide and the like have not been included as part of the electrolytecompositions during these evaluations, but their use is contemplated.These other hydroxides, as will be known to those familiar with thealkaline battery field, can often be advantageous for certain cellconfigurations.

Certain specific categories of inorganic electrolyte additives have beenfound, however, that further improve cell performance when used inconjunction with the electrochemically formed anode already described.These special additives form a part of this invention.

The initial group of such electrolyte additives involve the use ofmanganese compounds soluble in the alkali electrolyte solutions.Included in this group are potassium permanganate and zinc permanganate.Best results from a dendrite and shape change prevention standpoint havebeen obtained when these compounds have been added to electrolytes alsocontaining zincate in solution. Typical electrolyte formulationscontaining these manganese additives are shown in Table 3.

While not readily available for purchase, the potassium and zincmanganates also work as replacements for the permanganates. This hasbeen verified in several instances by successfully using potassiumpermanganate that had changed from purple to green color, indicating areduction from the +7 to the +6 potassium manganate oxidation state.

                  TABLE 2                                                         ______________________________________                                        KOH ELECTROLYTES WITH                                                         AND WITHOUT ZINCATE ADDITIONS                                                 ELECTROLYTE                                                                   TYPE                               PERCENT                                    DESIGNATION COMPOSITION  PERCENT   Zn                                         ______________________________________                                        25K         KOH          25        0                                                      H.sub.2 O                                                         30K         KOH          30        0                                                      H.sub.2 O                                                         35K         KOH          35        0                                                      H.sub.2 O                                                         40K         KOH          40        0                                                      H.sub.2 O                                                         25K28       KOH          25.0                                                             ZnO.sup.(a)  3.5       2.8                                                    H.sub.2 O                                                         30K35       KOH          30.0                                                             ZnO          4.3       3.5                                                    H.sub.2 O                                                         35K41       KOH          35.0                                                             ZnO          5.0       4.1                                                    H.sub.2 O                                                         30K44       KOH          30.0                                                             ZnO          5.4       4.4                                                    H.sub.2 O                                                         30K74       KOH          30.0                                                             ZnO          5.4                                                              Zn(OH).sub.2.sup.(b)                                                                       3.1       7.4                                                    H.sub.2 O                                                         ______________________________________                                         NOTES                                                                         .sup.(a) Zinc oxide, dry process, chemical grade, Certified A.C.S.            .sup.(b) Zinc hydroxide, chemically precipitated, washed and dried.      

                  TABLE 3                                                         ______________________________________                                        KOH ELECTROLYTES WITH                                                         SOLUBLE MANGANESE ADDITIONS                                                   ELECTROLYTE                      PER-  PER-                                   TYPE                     PER-    CENT  CENT                                   DESIGNATION COMPOSITION  CENT    Zn    Mn                                     ______________________________________                                        D-3         KOH          30.0                                                             KMnO.sub.4.sup.(c)                                                                         1.7     0     0.58                                               H.sub.2 O                                                         D-4/4       KOH          30.0                                                             ZnO.sup.(a)  4.3     3.45                                                     KMnO.sub.4   0.4           0.14                                               H.sub.2 O                                                         D-4/2       KOH          30.0                                                             ZnO          4.2     3.40                                                     KMnO.sub.4   0.8           0.28                                               H.sub.2 O                                                         D-4         KOH          30.0                                                             ZnO          4.2     3.40                                                     KMnO.sub.4   1.6           0.56                                               H.sub.2 O                                                         D-8         KOH          30.0                                                             ZnO          5.5     4.43                                                     KMnO.sub.4   1.6           0.55                                               H.sub.2 O                                                         D-8Z10      KOH          30.0                                                             ZnO          5.4                                                              Zn(OH).sub.2.sup.(b)                                                                       3.1     6.40                                                     KMnO.sub.4   1.5           0.53                                               H.sub.2 O                                                         D-2         KOH          25.0                                                             ZnO          3.0     2.43                                                     KMnO.sub.4   1.6           0.56                                               H.sub.2 O                                                         ______________________________________                                         NOTES                                                                         .sup.(a) Zinc oxide, dry process, chemical grade, certified A.C.S.            .sup.(b) Zinc hydroxide, chemically precipitated, washed and dried.           .sup.(c) Potassium pernanganate, reagent grade.                          

Other permanganates (and manganates) such as sodium, lithium, calcium,etc., should also make viable candidates, but the introduction of newion species into the electrolyte system should be taken intoconsideration.

When used in conjunction with the electrochemically oxidized anodes, aspreviously described, the inclusion of an alkali soluble manganese inthe electrolyte has been found to significantly reduce and, in manyinstances, to completely eliminate zinc dendrite formation. Associatedanode shape change is also noticeably minimized over that ofequivalently constructed cells tested without the manganese present.

The manganese is normally introduced into the cell as part of theoriginal electrolyte used during the initial electrochemical anodeoxidation process of the invention. By the time the anode has reachedits highly oxidized and expanded condition, the original purple or greencolor of the electrolyte has normally disappeared. This indicates thatthe manganese has been reduced to a lower oxidation state, such as thedioxide or hydroxide, and is now apparent only as a brownishdiscoloration at the anodes and separators and as a cloudy suspension inthe electrolyte.

In other tests, an excess of the alkali soluble manganese has beenplaced in the bottom of the cell to act as a reservoir of the highoxidation state material. While the purple electrolyte color remains inthis case, excellent dendrite protection was still afforded during cellcycling. Excellent results have also been obtained by introducing themanganese at the beginning of the initial charge cycle or even waitinguntil the first normal discharge cycle.

It is not known in what manner the manganese is entering into theelectrochemical system. There is a possibility of the formation of amanganese-zinc or manganese-zinc-potassium complex, since maximumdendrite prevention is not achieved unless ample zincate is also presentin the electrolyte.

Equally perplexing is the mechanism by which dendrite prevention andassociated shape change minimization is achieved. Our present theory isthat the manganese, whether as a compound or complex, is in some waypreventing or greatly reducing passivation at the zinc anode duringdischarge. That is, the manganese is inhibiting the formation of poorlyconductive zinc oxides. This would leave the anode in a highlyconductive state during the subsequent charge cycle, thereby enhancing amore uniform conversion of the oxidized zinc to the colloidal ormicrocrystalline metallic form within the oxide lattice. This sameeffect would, of course, also allow more uniform plating of zinc fromthe electrolyte should this occur.

The second group of electrolyte additives, considered to be a part ofthis invention, comprise the use of tin compounds soluble in theelectrolyte solution. More specifically, the tin compounds found usefulat this point included potassium stannate and zinc stannate.Representative electrolyte formulations containing these compounds areincluded in Table 4.

As in the case of the manganese electrolyte additives, these tinelectrolyte additives are also used in conjunction with the special,highly electrochemically oxidized anodes already described. Their usehas been found to noticeably reduce overall anode shape change, bothduring the special anode oxidizing process as well as during subsequentcharge/discharge cycling. The tin containing electrolyte additive may beused in combination with the manganese additive for the greatest overalldendrite and shape change prevention.

                                      TABLE 4                                     __________________________________________________________________________    KOH ELECTROLYTES WITH                                                         SOLUBLE TIN ADDITIONS                                                         ELECTROLYTE TYPE             PERCENT                                                                             PERCENT                                                                             PERCENT                              DESIGNATION  COMPOSITION                                                                             PERCENT                                                                             Zn    Mn    Sn                                   __________________________________________________________________________    S2           KOH       30.0                                                                ZnO.sup.(a)                                                                             4.3   3.46  0     0.25                                              K.sub.2 SnO.sub.3.3H.sub.2 O.sup.(d)                                                    0.6                                                    S6           KOH       30.0                                                                ZnO       4.2   3.40  0     0.75                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.9                                                    S1-D4        KOH       30.0                                                                ZnO       4.2   3.40                                                          KMnO.sub.4.sup.(c)                                                                      1.6         0.55  0.13                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            0.3                                                    S2-D4        KOH       30.0                                                                ZnO       4.2   3.40                                                          KMnO.sub.4                                                                              1.6         0.55  0.25                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            0.6                                                    S3-D4/2      KOH       30.0                                                                ZnO       4.2   3.40                                                          KMnO.sub.4                                                                              0.8         0.28  0.38                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.0                                                    S3-D4        KOH       30.0                                                                ZnO       4.2   3.40                                                          KMnO.sub.4                                                                              1.6         0.55  0.38                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.0                                                    S3-Z12       KOH       30.0                                                                ZnO       4.1   3.78  0.75  0.37                                              Zn(MnO.sub.4).sub.2.6H.sub.2 O.sup.(f)                                                  2.8                                                                 K.sub.2 SnO.sub.3.3H.sub.2 O                                                            0.9                                                    S6-D4/2      KOH       30.0                                                                ZnO       4.2   3.38                                                          KMnO.sub.4                                                                              0.8         0.27  0.75                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.9                                                    S6-D4        KOH       30.0                                                                ZnO       4.2   3.34                                                          KMnO.sub.4                                                                              1.6         0.54  0.75                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.9                                                    S6-D8        KOH       30.0                                                                ZnO       5.4   4.35                                                          KMnO.sub.4                                                                              1.5         0.54  0.74                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.9                                                    S6-Z12       KOH       30.0                                                                ZnO       4.1   3.74  0.74  0.74                                              Zn(MnO.sub.4).sub.2.6H.sub.2 O                                                          2.8                                                                 K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.9                                                    S9-D4/2      KOH       30.0                                                                ZnO       4.2   3.34                                                          KMnO.sub.4                                                                              0.8         0.27  1.11                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            2.8                                                    S9-D4        KOH       30.0                                                                ZnO       4.1   3.31                                                          KMnO.sub.4                                                                              1.6         0.54  1.11                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            2.8                                                    S9-Z12       KOH       30.0                                                                ZnO       4.1   3.71  0.73  1.09                                              Zn(MnO.sub.4).sub.2.6H.sub.2 O                                                          2.8                                                                 K.sub.2 SnO.sub.3.3H.sub.2 O                                                            2.8                                                    ZS6-D3       KOH       30.0                                                                KMnO.sub.4                                                                              1.6   0.42  0.57  0.76                                              ZnSnO.sub.3.4H.sub.2 O.sup.(a)                                                          2.0                                                    ZS6-D8       KOH       30.0                                                                ZnO       5.4                                                                 KMnO.sub.4                                                                              1.5   4.75  0.53  0.72                                              ZnSnO.sub.3.4H.sub.2 O                                                                  1.9                                                    ZS24-D3      KOH       30.0                                                                KMnO.sub.4                                                                              1.5   1.59  0.53  2.88                                              ZnSnO.sub.2.4H.sub.2 O                                                                  7.4                                                    ZS30-D3      KOH       30.0                                                                KMnO.sub.4                                                                              1.5   1.95  0.53  3.55                                              ZnSnO.sub.3.4H.sub.2 O                                                                  9.1                                                    S6ZS6-D8Z10  KOH       30.0                                                                ZnO       5.3                                                                 Zn(OH).sub.2.sup.(b)                                                                    3.0                                                                 KMnO.sub.4                                                                              1.5   6.58  0.52  1.41                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.8                                                                 ZnSnO.sub.3.4H.sub.2 O                                                                  1.8                                                    ZS6-D8Z10    KOH       30.0                                                                ZnO       5.3                                                                 Zn(OH).sub.2                                                                            3.0                                                                 KMnO.sub.4                                                                              1.5   6.58  0.52  0.70                                              ZnSnO.sub.3.4H.sub.2 O                                                                  1.8                                                    S6-D8Z10     KOH       30.0                                                                ZnO       5.3                                                                 Zn(OH).sub.2                                                                            3.0                                                                 KMnO.sub.4                                                                              1.5   6.19  0.52  0.71                                              K.sub.2 SnO.sub.3.4H.sub.2 O                                                            1.8                                                    S6ZS6-D8     KOH       30.0                                                                ZnO       5.2                                                                 KMnO.sub.4                                                                              1.5   4.53  0.51  1.40                                              K.sub.2 SnO.sub.3.3H.sub.2 O                                                            1.8                                                                 ZnSnO.sub.3.4H.sub.2 O                                                                  1.8                                                    __________________________________________________________________________     NOTES                                                                         .sup.(a) Zinc oxide, dry process, chemical grade, Certified A.C.S.            .sup.(b) Zinc hydroxide, chemically precipitated, washed and dried.           .sup.(c) Potassium permanganate, reagent grade.                               .sup.(d) Potassium stannate, 95%                                              .sup.(e) Zinc stannate, chemically precipitated, washed and dried.            .sup.(f) Zinc permanganate, high purity grade.                           

The two additives appear to function virtually independently of oneanother, the tin having only a minor effect on the dendrite prevention,but both helping in regard to the initial shape change phenomena.

Once converted to its metallic form, the tin, unlike the zinc in theanode, should remain in this reduced form on subsequent dischargecycling. Microscopic examination of such anodes after cycling indicatethat the tin has become uniformly distributed within the expanded anodestructure. This can often be seen as thin metallic flakes of a silverycolor. Anodes containing the tin also result in a stronger, more uniformbut less expanded anode structure than when processed with similarelectrolytes not containing the tin. The improvement noticed in anodeshape change with the involvement of the tin may be strictly mechanicalin nature, may result from an increase in electrical conductivity withinthe anode structure, or may be related to the formation of permanentsites for mercury retention as suggested by Kamai and Uchida in U.S.Pat. No. 3,617,384.

Related tin containing compounds, such as sodium stannate, cupricstannate, lead stannate and the like stannate compounds are alsoexpected to be useful.

In testing of cells using the highly electrochemically oxidized anodesof this invention, it has been found to be important to include zincatewith the potassium hydroxide electrolyte. This is true whether or notthe electrolyte solutions also contain manganese and/or tin additions.While dendrite protection can be afforded with moderate zincateadditions, large concentrations have been found to provide excellentdendrite prevention in addition to minimum anode shape change.

When electrochemical oxidation of the special anodes has been attemptedwithout zincate containing electrolytes, a considerable amount of anodedistortion and severe surface irregularities have results. This is nodoubt due to the fact that zincate now can only be supplied to theelectrolyte directly from the anode itself. The surface distortion maybe due to the rapid transfer of zinc from the anode to the electrolytebut also may be connected with a preferential transfer from certainanode surface areas that have become wet with electrolyte faster thanothers.

In the electrolyte formulations of Tables 2, 3 and 4, the zincate hasnormally been added by simply dissolving specified amounts of a chemicalgrade of zinc oxide (ZnO) in a KOH solution. Zinc hydroxide can also beused as well as other zinc compounds including potassium zincate. Thezincate can also be introduced by the electrochemical oxidation ofmetallic zinc electrodes placed in a potassium hydroxide solution.

Another important variation of this invention involves the inclusion ofa selected group of materials in the anode prior to its specialelectrochemical oxidation step. These anode additives are all inorganicand are included in the slurry mixture from which the initial anode ismade. The specific anode additives that have been found to be of mostvalue are zinc stannate, stannic oxide and finely divided tin metal.Potassium stannate can also be used in small amounts or in combinationwith the other tin additives such as zinc stannate. It is difficult touse alone, however, because it tends to deflocculate the water basedslurry.

As in the case of electrolytes containing tin in solution, the tinincorporated in the anode has been found to minimize initial and longerterm shape change phenomena. These anodes are normally used with zincatecontaining electrolytes that also include the soluble manganese additivefor minimizing dendrite formation. The electrolyte may also containsoluble tin.

In addition to the tin anode additive just disclosed, the addition offinely divided lead or copper metal powder was also found to provide aminor but noticeable improvement in overall anode shape change. Examplesof anode slurry formulations containing these additives are included inTable 1.

It has been found that anode additives consisting of other stannatessuch as copper and lead are also of value. It is expected that otherstannates will be useful.

The test cells employed in the example of this disclosure are shownschematically in FIGS. 1, 2 and 3. This construction utilizes a singlenickel cathode 10 centrally positioned between two zinc anodes 12.Porous nylon separators 14 are placed between the electrodes and alsobetween each anode and the polyethylene envelope 16 that serves as thecell "container".

After the insertion of the electrode separator assembly, thepolyethylene envelope is heat sealed across the open end, as at 20. Theelectrode electrical leads 17 and 18 have been previously coated with athin layer of tar so that they are also effectively sealed to theenvelope at the same time. The electrolyte is then added to the sealedenvelope by means of a syringe with a small needle. The needle isinserted through the envelope near the top and the resulting small holeserves as the cell vent during the special electrochemical anodeoxidizing step and subsequent cell charge/discharge cycling.

The cathode employed in these experimental test cells were made fromlarger nickel cathodes removed from unused aircraft-type nickel/cadmiumcells. All cathodes came from a specific type battery made by the samemanufacturer. The new cathodes were carefully cut to size, the excessnickel oxide coating removed from the tab to expose the nickel currentcollector and a nickel strip then spot welded to this tab to serve asthe electrical lead. Following the cathode preparation, they arecarefully cleaned by means of boiling water treatments.

The anodes used in this cell configuration were constructed as halfanodes like those that might be used as the outside electrodes ofmulticell batteries. That is, the zinc coating was applied to one sideof the current collector only and is installed so that the coated sideof each anode is facing the cathode.

This method was selected as being the most nearly identical to that of amulticell configuration, while at the same time allowing the use of astandard nickel cathode that could be obtained from commerciallyavailable batteries.

During the initial anode electrochemical oxidation step and subsequentcharge/discharge cycling, the cell is securely held, under pressure, ina multicell holding fixture. This is accomplished by placing theindividual cells between synthetic sponge rubber pads and tightening theassembly by means of an adjustable end plate.

The separators, as explained earlier, are of non-woven material(normally nylon) and measure 3.97 cm by 4.45 cm and are approximately0.023 cm in thickness. In most of the experimental cells of thisdisclosure, three separators were used between the electrodes so thatthe anode-to-cathode separation is approximately 0.075 cm. Twoadditional separators are placed between the back side of each anode andthe polyethylene envelope to assure adequate electrolyte contact andwicking. During assembly, the electrodes are centrally positioned on theflat surface area of the separators. No separator wrapping of theelectrodes is involved and no microporous membranes have been included.

The cathode dimensions used in the test cells of this disclosure areapproximately 2.7 cm wide by 3.2 cm in length by 0.08 cm thick. Thesehave been cut from the larger nickel cathodes of 10 ampere-hournickel/cadmium batteries. From tests made with the 10 ampere-hourbatteries, and allowing for minor variations between electrodes, it hasbeen determined that these small cathodes have a nominal capacity of0.25 ampere hours. Since the cathode is the limiting electrode, theoverall cell capacity is also in the order of 0.25 ampere hours.

The initial anode dimensions are slightly smaller than those of thecathode and measure approximately 2.6 cm wide by 3.1 cm long by 0.06 cmin thickness; however, this initial anode size may undergo anappreciable expansion during the preliminary electrochemical anodeconversion step. The expanded anode may typically measure 3.2 cm by 3.7cm by as much as 1.5 cm in thickness.

The current collector, aside from its electrical lead, has the samewidth and height dimensions as the initial anode. Following the anodepasting operation, and allowing to air dry, the excess zinc around theedges of the anode has been removed by sanding flush with the edges ofthe current collector.

The testing of the experimental nickel/zinc cells referred to in thisapplication has been exclusively carried out using well regulated powersupplies. The constant current charging and discharging of the cells hasbeen controlled by timers. In addition, a transistorized circuit hasbeen employed for each cell to cut off the constant current discharge ata pre-set voltage level of 0.9 volts (or almost at complete discharge).In this way, virtually full charge and discharge cycling has beenachieved without the need of monitoring to assure that specific cellsare not being over or under charged due to shifts in electrodeacceptance levels. A complete discharge of each cell is assured bysimply using a discharge time that is sufficiently long, at the selectedconstant current discharge rate, for the most efficient cell beingtested to reach the 0.9 volt level.

EXAMPLES OF THE INVENTION

Examples of the experimental nickel/zinc cells, using a variety of testconfigurations, are contained in the following tables of thisdisclosure. These examples will further serve to illustrate theinvention.

Table 6 covers cells using KOH electrolyte, both containing and notcontaining zincate in solution. Also, included are a number of exampleswhere alkali soluble manganese has been added to such electrolytes.

Table 7 lists test cells run with KOH electrolytes, such as those ofTable 6, to which alkali soluble tin compounds have additionally beenincluded.

Each of the cells in these two tables have been initially assembledusing anodes containing only zinc dust and mercuric oxide as the activeingredients. This anode formulation has been designated as type I. Theslurry composition for this anode has been previously set forth in Table1.

Table 8 includes cells in which selected inorganic materials have beenincluded in the anode formulation in addition to the zinc and mercuricoxide. These cells have been tested with a variety of KOH electrolytes,with or without zincate, manganese and tin. Again, the specific slurryformulations for each of the anode types listed in Table 8 can bedetermined by referring to Table 1.

Each of the cells included in Tables 6, 7 and 8 have undergone thespecial electrochemical anode oxidation step of this invention prior tothe start of the charge/discharge cycling. In each case, this has beenaccomplished in the fully assembled cell, with the electrolyte present,by simply electrically shorting the electrode terminals with a lowresistance connection. A total shorting time of 22 hours has been usedfor this initial anode oxidation step unless otherwise noted.

Chemical analysis has shown that after the 22 hour shorting of anodesfrom a cell such as #123-2 in Table 6, about 96% of the anode has beenconverted to an oxidised form with the remaining about 4% still asmetallic zinc.

    TABLE 6      TEST CELL EXAMPLES WITH AND WITHOUT ZINCATE AND MANGANESE INCLUDED WITH     KOH ELECTROLYTE [ANODES ELECTROCHEMICALLY OXIDIZED ACCORDING TO METHOD     OF THE INVENTION PRIOR TO ELECTRICAL CELL CYCLING]    Per-  Cur-     Electro-   Per-       Dendrite   Anode  cent  rent lyte   cent     Dis-     Penetration  Type Major Addi-  Col- Type  Major Elec-  Initial   charge     Cell Thru Test (See Anode tive in Anode lector (See Per- Electro-     trolyte Separa- Cathode Cell Cell Measur- Disas- Separators Anode Shape     Change.sup.(g) Cell Table Addi- Slurry Weights.sup.(a) Mate- Table cent     lyte Addi- tor Con- Charge Discharge ed at sembled Indicated.sup.(e)     Notations and Special # 1) tives Mix (grams) rial.sup.(b) 2&3) KOH     Additives tive System.sup.(c) dition.sup.(d) Cycle.sup.(f) Cycle.sup.(f) C     ycle # at Cycle # 1 2 3  Remarks       64-3 I HgO 2.5 1.75 Ni 30K35 30 ZnO 4.3 N-2 D 6 hrs @ 1.57 hrs   4 0 0          1.70        42 ma 150 ma 32 34 5 0 0 some shape change     (0.252Ah) (0.235Ah) 64-4 I HgO 2.5 1.75 Ni D-4 30 ZnO 4.2 N-2 D 6 hrs @     1.62 hrs   0 0 0 very little shape change     1.70    KMnO.sub.4 1.6     42 ma 150 ma 32 34 0 0 0 definitely less than 64-3             (0.252Ah)     (0.243Ah) 64-1 I HgO 2.5 1.75 Ni 30K35 30 ZnO 4.3 N-2 C 6 hrs @ 1.6 hrs      2 0 0 some shape change     1.85        42 ma 150 ma 32 34 5 0 0     ˜ 64-1             (0.252Ah) (0.240Ah) 64-2 I HgO 2.5 1.75 Ni D-4     30 ZnO 4.2 N-2 C 6 hrs @ 1.6 hrs   0 0 0 very little shape change     1.80    KMnO.sub.4 1.6   42 ma 150 ma 32 34 0 0 0 ˜ 64-4       (0.252Ah) (0.240Ah) 71-1 I HgO 2.5 1.75 Ni 30K35 30 ZnO 4.3 N-2 D 2     hrs @    10 0 0 some shape change     1.70        100 ma    7 0 0          4 hrs @ 150 ma 33 36             12.5 ma             (0.250Ah) 71-2     I HgO 2.5 1.65 Ni D-4 30 ZnO 4.2 N-2 D 2 hrs @ 1.52 hrs   0 0 0 very     little shape change     1.65    KMnO.sub.4 1.6   100 ma 150 ma 33 36 0 0     0 definitely less than 71-1             4 hrs @ (0.228Ah)     12.5 ma             (0.250Ah) 71-3 I HgO 2.5 1.75 Ni 30K 30 none -- N-2     D 2 hrs @ 1.43 hrs   7 0 0 anode badly wrinkled and     1.8        100     ma 150 ma 33 36 X 3 0 erroded-considerable             4 hrs @ (0.215Ah)          shape change             12.5 ma             (0.250Ah) 71-4 I HgO     2.5 1.7 Ni D-3 30 KMnO.sub.4 1.6 N-2 D 2 hrs @ 1.58 hrs   F 5 0 anode     badly wrinkled and     1.7        100 ma 150 ma 33 36 X 7 0 erroded-consi     derable             4 hrs @ (0.238Ah)      shape change, ˜ 71-3              12.5 ma             (0.250Ah) 76-1 I HgO 2.5 1.7 Cu 30K35 30     ZnO 4.3 N-3 D 6 hrs @ 1.42 hrs   0 0 0 some shape change     1.8 Plated          38.5 ma 150 ma 39 48 0 0 0      Ni       (0.231Ah) (0.213Ah) 76-2 I     HgO 2.5 1.7 Cu D-4 30 ZnO 4.2 N-3 D 6 hrs @ 1.45 hrs   0 0 0 a little     less shape     1.85 Plated   KMNO.sub.4 1.6   38.5 ma 150 ma 39 48 0 0 0     change than 76-1      Ni       (0.218Ah) (0.218Ah) 78-1 I HgO 2.5 1.9 Ni     D-4 30 ZnO 4.2 N-3 D 6 hrs @ 1.47 hrs   0 0 0     1.8    KMnO.sub.4 1.6      38.5 ma 150 ma 33 41 0 0 0 very little shape change     (0.231Ah) (0.220Ah)      Ag 78-2 I HgO 2.5 1.8 over D-4 30 ZnO 4.2 N-3 D     6 hrs @ 1.48 hrs   0 0 0     1.95 Cu   KMnO.sub.4 1.6   38.5 ma 150 ma     33 41 0 0 0 very slightly more shape      Plated       (0.231Ah)     (0.222Ah)      change than 78-1      Ni      Sn 78-3 I HgO 2.5 1.85 over     D-4 30 ZnO 4.2 N-3 D 6 hrs @ 1.48 hrs   0 0 0     1.80 Cu   KMnO.sub.4     1.6   38.5 ma 150 ma 33 41 0 0 0 shape change ˜ 78-2      Plated         (0.231Ah) (0.222Ah)      Ni 79-7 I HgO 2.5 1.75 Cu D-4/2 30 ZnO 4.2     N-3 D 6 hrs @ 1.56 hrs   0 0 0     1.75 Plated   KMnO.sub.4 0.8   40 ma     150 ma 33 36 0 0 0 very little shape change      Ni       (0.240Ah)     (0.234Ah) 80-1 I HgO 2.5 1.9 Ni 30K35 30 ZnO 4.3 N-3 D 10 hrs @ 1.42 hrs       0 0 0     1.8        25 ma 150 ma 30 34 1 0 0 a little shape change              (0.250Ah) (0.213Ah) 80-2 I HgO 2.5 1.85 Ni D-4 30 ZnO 4.2 N-3 D     10 hrs @ 1.58 hr   0 0 0     1.9    KMnO.sub.4 1.6   25 ma 150 ma 30 34     0 0 0 very little shape change             (0.250Ah) (0.236Ah) 80-3 I     HgO 2.5 1.95 Cu 30K35 30 ZnO 4.3 N-3 D 10 hrs @ 1.46 hr   4 0 0     2.05     Plated       25 ma 150 ma 30 34 0 0 0 a little shape change      Ni      (0.250Ah) (0.219Ah)      80-1 80-4 I HgO 2.5 1.90 Cu D-4 30 ZnO 4.2 N-3     D 10 hrs @ 1.51 hr   0 0 0     1.85 Plated   KMnO.sub.4 1.6   25 ma 150     ma 30 34 0 0 0 very little shape change      Ni       (0.250Ah) (0.226Ah)           ˜ 80-2      Sn 80-5 I HgO 2.5 2.2 over 30K35 30 ZnO 4.3 N-3     D 10 hrs @ 1.47 hr   2 0 0     2.1 Cu       25 ma 150 ma 30 34 0 0 0 a     little shape change      Plated       (0.250Ah) (0.220Ah)      ˜     80-1 and 80-3      Ni      Sn 80-6 I HgO 2.5 1.85 over D-4 30 ZnO 4.2     N-3 D 10 hrs @ 1.42 hr   0 0 0     1.85 Cu   KMnO.sub.4 1.6   25 ma 150     ma 30 34 0 0 0 very little shape change      Plated       (0.250Ah)     (0.213Ah)      ˜ 80-2 and 80-4      Ni 80-7 I HgO 2.5 1.8 Cu D-4/2     30 ZnO 4.2 N-3 D 10 hrs @ 1.55 hr   0 0 0     1.8 Plated   KMnO.sub.4     0.8   25 ma 150 ma 30 34 0 0 0 very little shape change      Ni     (0.250Ah) (0.233Ah)      80-4 80-8 I HgO 2.5 1.8 Cu D-4/4 30 ZnO 4.3 N-3     D 10 hrs @ 1.5 hr   0 0 0     1.85 Plated   KMnO.sub.4 0.4   25 ma 150     ma 30 34 0 0 0 a little shape change ˜      Ni       (0.250Ah)     (0.225Ah)      more than 80-7, a little                    less than     80-3 92-10 I HgO 2.5 1.45 Cu D-4 30 ZnO 4.3 N-3 D 10 hrs 2 1.45 hr   0 0     0     1.5 Plated +  KMnO.sub.4 excess   25 ma 150 ma 32 38 0 0 0 very     little shape change      Ni excess      (0.250Ah) (0.218Ah)     KMnO.sub.4       crystals 103-1 I HgO 2.5 1.55 Cu D-2 25 ZnO 3.0 S-3 D     10 hrs @ 1.08 hr   0 0 0     1.55 Plated   KMnO.sub.4 1.6   25 ma 150 ma     25 31 0 0 0 very low cell output-      Ni       (0.250Ah) (0.162Ah)     typical of low KOH                    concentrations, almost no               shape change 119-5 I HgO 2.5 1.55 Cu D8Z10 30 ZnO 5.4 N-3 D 10     hrs @ 1.48 hrs   0 0 0     1.55    Zn(OH).sub.2 3.1   23 ma 150 ma 39 41     0 0 0 almost no shape change         KMnO.sub.4 1.5   (0.230Ah) (0.223Ah)           extremely uniform anode                 5 0 0 123-1 I HgO 2.5 1.45     Cu 30K35 30 ZnO 4.3 N-3 D 10 hrs @ 1.43 hrs 20 34 0 0 0 some shape     change     1.5        23 ma 150 ma             (0.230Ah) (0.215Ah)               0 0 0 123-2 I HgO 2.5 1.45 Cu D-4 30 ZnO 4.2 N-3 D 10 hrs @     1.47 hrs 20 34 0 0 0 definitely less shape     1.45    KMnO.sub.4 1.6     23 ma 150 ma      change than 123-1             (0.230Ah) (0.220Ah)     NOTES     .sup.(a) Weight in grams is listed for each of the two anodes in each     cell. Weight is for the dried slurry composition but does not include the     weight of the current collector.     .sup.(b) Current collectors are copper, nickel or electroplated nickel as     indicated. All collectors are made of a fine mesh expanded metal, type     74/0, manufactured by Exmet Corporation, Bridgeport, Connecticut.     .sup.(c) Separator system is as shown in Table 5 and accompanying     description. N2 indicates use of a nonwoven nylon type 2505K4 with random     fiber orientation and N3 indicates type 2503K4 with unidirectional fiber     orientation. Both types are sold under the trade name PELLON and are     manufactured by the Pellon Corporation, Lowell, Massachusetts.     .sup.(d) Initial cathode condition as installed in the cell is indicated     as "C" for fully charged or as "D" for fully discharged.     .sup.(e) Separator #1 is that closest to the anode, #2 is the centrally     located separator and #3 is the separator facing against the cathode. A     number in the separator column indicates that specific number of dendrite     as having penetrated to or thru the particular separator. The letter X     indicates many dendrites and F indicates a few.     .sup.(f) Constant current charging and discharging were employed.     .sup.(g) Unless otherwise noted, shape change notations indicate a     widening and thickening of the expanded anode material increasing in     magnitude from the top toward the bottom. All cells have been run with th     anodes in a vertical position.

    TABLE 7      TEST CELL EXAMPLES USING TIN COMPOUNDS INCLUDED WITH KOH BASED ELECTROLY     TE [ANODES ELECTROCHEMICALLY OXIDIZED ACCORDING TO METHOD OF THE     INVENTION PRIOR TO ELECTRICAL CELL CYCLING]    Per-  Cur-    Per-     Dendrite   Anode  cent  rent Electro-   cent  Initial   Dis- Cell     Penetration  Type Major Addi-  Col- lyte   Elec- Sepa- Cath-   charge     Disas- Thru Test (See Anode tive in Anode lector Type Per- Major trolyte     rator ode Cell Cell Measur- sembled Separators Anode Shape Change.sup.(g)      Cell Table Addi- Slurry Weights.sup.(a) Mate- (See cent Electrolyte     Addi- Sys- Con- Charge Discharge ed at at Indicated.sup.(e) Notations     and Special # 1) tives Mix (grams) rial.sup.(b) Table 4) KOH Additives     tive tem.sup.(c) dition.sup.(d) Cycle.sup.(f) Cycle.sup.(f) Cycle #     Cycle # 1 2 3 Remarks         73-3 I HgO 2.5 1.8 Ni S2 30 ZnO 4.3 N-3 D 6 hrs @ 1.52 hrs   F 0 0 a     little shape change     1.85    K.sub.2 SnO.sub.3 0.6   40 ma 150 ma 34     36 3 0 0             (0.240Ah) (0.228Ah)  73-4 I HgO 2.5 1.9 Ni S2-D4 30     ZnO 4.2 N-3 D 6 hrs @ 1.55 hrs   0 0 0 very little shape change     1.8       KMnO.sub.4 1.6   40 ma 150 ma 34 36 0 0 0         K.sub.2 SnO.sub.3     0.6   (0.240Ah) (0.233Ah)  95-1 I HgO 2.5 1.35 Ni S2 30 ZnO 4.3 N-3 D 10     hrs @ 1.49 hrs   0 0 0 a little shape change     1.3    K.sub.2      SnO.sub.3 0.6   23 ma 150ma 25 34 0 0 0             (0.230Ah) (0.230Ah)     111-1 I HgO 2.5 1.4 Cu S6-D4 30 ZnO 4.2 N-3 D 10 hrs @ 1.5 hrs   0 0 0     very little shape change     1.5    KMnO.sub.4 1.6   23 ma 150 ma 24 30     0 0 0         K.sub.2 SNO.sub.3 1.9   (0.230Ah) (0.225Ah) 111-2 I HgO     2.5 1.5 Cu S2-D4 30 ZnO 4.2 N-3 D 10 hrs @ 1.42 hrs   0 0 0 very little     shape change     1.55    KMnO.sub.4 1.6   23 ma 150 ma 24 30 0 0 0 111-1             K.sub.2 SnO.sub.3 0.6   (0.230Ah) (0.213Ah) 111-3 I HgO 2.5 1.45     Cu S1-D4 30 ZnO 4.2 N-3 D 10 hrs @ 1.43 hrs   0 0 0 very slightly more     shape     1.55    KMnO.sub.4 1.6   23 ma 150 ma 24 30 0 0 0 change than     111-1 and 111-2         K.sub.2 SnO.sub.3 0.3   (0.230Ah) (0.214Ah)     111-4 I HgO 2.5 1.5 Cu S2 30 ZnO 4.3 N-3 D 10 hrs @ 1.37 hr   0 0 0     definitely more shape change     1.55    K.sub.2 SnO.sub.3 0.6   23 ma     150 ma 24 30 0 0 0 than 111-2 & 111-3 where             (0.230Ah)     (0.205Ah)      manganese additive was                    present in     electrolyte 115-4 I HgO 2.5 1.60 Cu S3-D4/2 30 ZnO 4.2 N-3 D 10 hrs @     1.48 hrs   0 0 0     1.5    KMnO.sub.4 0.8   23 ma 150 ma 29 31 0 0 0 a     little shape change         K.sub.2 SnO.sub.3 1.0   (0.230Ah) (0.221Ah)     115-5 I HgO 2.5 1.6 Cu S3-D4 30 ZnO 4.2 N-3 D 10 hrs @ 1.44 hrs   0 0 0        1.5    KMnO.sub.4 1.6   23 ma 150 ma 29 31 0 0 0 very slightly less     shape         K.sub.2 SnO.sub.3 1.0   (0.230Ah) (0.216Ah)      change     than 113-4 115-6 I HgO 2.5 1.6 Cu S3-Z12 30 ZnO 4.1 N-3 D 10 hrs @ 1.44     hrs   0 0 0     1.5    KMnO.sub.4 2.8   23 ma 150 ma 29 31 0 0 0 shape     change ˜ 115-5         K.sub.2 SnO.sub.3 0.9   (0.230Ah) (0.216Ah)     116-1 I HgO 2.5 1.55 Cu S6-D4/2 30 ZnO 4.2 N-3 D 10 hrs @ 1.5 hr   0 0 0         1.55    KMnO.sub.4 0.8   23 ma 150 ma 27 31 0 0 0 a little shape     change         K.sub.2 SnO.sub.3 1.9   (0.230Ah) (0.225Ah) 116-2 I HgO     2.5 1.55 Cu S6-D4 30 ZnO 4.2 N-3 D 10 hrs @ 1.48 hr   0 0 0     1.55     KMnO.sub.4 1.6   23 ma 150 ma 27 31 0 0 0 overall shape change ˜     116-1         K.sub.2 SnO.sub.3 1.9   (0.230Ah) (0.223Ah) 116-3 I HgO     2.5 1.55 Cu S6-Z12 30 ZnO 4.1 N-3 D 10 hrs @ 0.5 hr   0 0 0     1.55     Zn(MnO.sub.4).sub.2 2.8   23 ma 150 ma 27 31 0 0 0 overall shape change     ˜ 116-1         K.sub.2 SnO.sub.3 1.9   (0.230Ah) (0.225Ah)     and 116-2 116-4 I HgO 2.5 1.6 Cu S9-D4/2 30 ZnO 4.2 N-3 D 10 hrs @ 1.46     hr   0 0 0     1.5    KMnO.sub.4 0.8   23 ma 150 ma 27 31 0 0 0 overall     shape change ˜ 116-1         K.sub.2 SnO.sub.3 2.8   (0.230Ah)     (0.219Ah)      116-2 and 116-3                    a few tin flecks     visible                    in all three separators 116-5 I HgO 2.5 1.6     Cu S9-D4 30 ZnO 4.1 N-3 D 10 hrs @ 1.49 hr   0 0 0     1.5    KMnO.sub.4     1.6   23 ma 150 ma 27 31 0 0 0 shape change ˜ 116-4     K.sub.2 SnO.sub.3 2.8   (0.230Ah) (0.224Ah)      a few tin flecks     visible                    in 1st separator only 116-6 I HgO 2.5 1.5 Cu     S9-Z12 30 ZnO 4.1   10 hrs @ 1.46 hr   0 0 0     1.6    Zn(MnO.sub.4).sub     .2 2.8 N-3 D 23 ma 150 ma 27 31 0 0 0 Shape change ˜ 116-4     K.sub.2 SnO.sub.3 2.8   (0.230Ah) (0.219Ah)      almost no tin flecks                     visible in separators 117-4 I HgO 2.5 1.5 Cu S6-D8 30     ZnO 5.4 N-3 D 10 hrs @ 1.43 hr   0 0 0     1.6    KMnO.sub.4 1.5   23 ma     150 ma 31 33 0 0 0 very little shape change         K.sub.2 SnO.sub.3     1.9   (0.230Ah) (0.215Ah) 118-1 I HgO 2.5 1.5 Cu ZS6-D3 30 KMnO.sub.4     1.6 N-3 D 10 hrs @ 1.48 hr   6 0 0     1.5    ZnSnO.sub.3 2.0   23 ma     150 ma 29 31 8 0 0 extremely bad shape change             (0.230Ah)     (0.223Ah)      and some holes in anodes 118-2 I HgO 2.5 1.5 Cu ZS30-D3     30 KMnO.sub.4 1.5 N-3 D 10 hrs @ 1.42 hr   1 0 0 anode very irregular in         1.5    ZnSnO.sub.3 9.1   23 ma 150 ma 29 31 0 0 0 shape and a great     many holes             (0.230Ah) (0.213Ah)      in the expanded structure                         a large number of tin flecks     visible in separators 118-5 I HgO 2.5 1.5 Cu ZS6-D8 30 ZnO 5.4 N-3 D 10     hrs @ 1.48 hr   0 0 0     1.5    KMnO.sub.4 1.5   23 ma 150 ma 29 31 0 0     0 much less shape change than         ZnSnO.sub.3 1.9   (0.230Ah)     (0.223Ah)      zincate free 118-1 more                    shape change     than 117-4                    with K.sub.2 SnO.sub.3 119-1 I HgO 2.5 1.5     Cu ZS6- 30 ZnO 5.3 N-3 D 10 hrs @ 1.44 hr   0 0 0     1.55  D8Z10     Zn(OH).sub.2 3.0   23 ma 150 ma 39 41 0 0 0 very little shape change         KMnO.sub.4 1.5   (0.230Ah) (0.216Ah)         ZnSnO.sub.3 1.8 119-3 I     HgO 2.5 1.55 Cu S6- 30 ZnO 5.3 N-3 D 10 hrs @ 1.46 hr   0 0 0     1.5     D8Z10  Zn(OH).sub.2 3.0   23 ma 150 ma 39 41 0 0 0 very little shape     change         KMnO.sub.4 1.5   (0.230Ah) (0.220Ah)      ˜ 119-1           K.sub.2 SnO.sub.3 1.8 119-4 I HgO 2.5 1.5 Cu S6ZS6- 30 ZnO 5.2 N-3     D 10 hrs @ 1.46 hrs   0 0 0     1.55  D8  KMnO.sub.4 1.5   23 ma 150 ma     39 41 0 0 0 very little shape         K.sub.2 SnO.sub.3 1.8   (0.230Ah)     (0.220Ah)      ˜ 119-1 and 119-2         ZnSnO.sub.3 1.8 123-3 I     HgO 2.5 1.55 Cu S-6 30 ZnO 4.2 N-3 D 10 hrs @ 1.41 hrs   0 0 0     1.4      K.sub.2 SnO.sub.3 1.9   23 ma 150 ma 18 34 0 0 0 a little shape change                (0.230Ah) (0.211Ah) 123-4 I HgO 2.5 1.45 Cu S6-D4 30 ZnO 4.2     N-3 D 10 hrs @ 1.41 hrs   0 0 0     1.55    KMnO.sub.4 1.6   23 ma 150     ma 18 34 0 0 0 very little shape change         K.sub.2 SnO.sub.3 1.9     (0.230Ah) (0.211Ah)     NOTES     .sup.(a) Weight in grams is listed for each of the two anodes in each     cell. Weight is for the dried slurry composition but does not include the     weight of the current collector.     .sup.(b) Current collectors are copper, nickel or electroplated nickel as     indicated. All collectors are made of a fine mesh expanded metal, type     74/0, manufactured by Exmet Corporation, Bridgeport, Connecticut.     .sup.(c) Separator system is as shown in Table 5 and accompanying     description. N2 indicates use of nonwoven nylon type 2505K4 with random     fiber orientation and N3 indicates type 2503K4 with unidirectional fiber     orientation. Both types are sold under the trade name PELLON and are     manufactured by the Pellon Corporation, Lowell, Massachusetts.     .sup.(d) Initial cathode condition as installed in the cell is indicated     as "C" for fully charged or as "D" for fully discharged.     .sup.(e) Separator #1 is that closest to the anode, #2 is the centrally     located separator and #3 is the separator facing against the cathode. A     number in the separator column indicates that specific number of dendrite     as having penetrated to or thru the particular separator. The letter X     indicates many dendrites and F indicates a few.     .sup.(f) Constant current charging and discharging were employed.     .sup.(g) Unless otherwise noted, shape change notations indicate a     widening and thickening of the expanded anode material increasing in     magnitude from the top toward the bottom. All cells have been run with th     anodes in a vertical position.

    TABLE 8      TEST CELL EXAMPLES USING ANODE FORMULATIONS CONTAINING TIN COMPOUNDS     AND OTHER SELECTED ADDITIONS [ANODES ELECTROCHEMICALLY OXIDIZED ACCORDING      TO THE METHOD OF THE INVENTION PRIOR TO ELECTRICAL CELL CYCLING]     Per-  Cur- Electro-   Per-       Dendrite   Anode  cent  rent lyte     cent     Dis- Cell Penetration  Type  Addi-  Col- Type  Major Elec-     Initial   charge Disas- Thru Test (See Major tive in Anode lector (See     Per- Electro- trolyte Separa- Cathode Cell Cell Meas- sembled Separators     Anode Shape Change.sup.(g) Cell Table Anode Slurry Weights.sup.(a) Mate-     Table cent lyte Addi- tor Con- Charge Discharge ured at at Indicated.sup.     (e) Notations and Special # 1) Additives Mix (grams) rial.sup.(b) 2&3&4) K     OH Additives tive System.sup.(c) dition.sup.(d) Cycle.sup.(f) Cycle.sup.(     f) Cycle # Cycle # 1 2 3  Remarks       120-3 EW HgO 2.5 1.5 Cu D-8Z10 30 ZnO 5.4 N-3 D 10 hrs @ 1.49 hr   0 0 0         ZnSnO.sub.3 2.0 1.55    Zn(OH).sub.2 3.1   23 ma 150 ma 37 39 0 0 0     no discernable shape         KMnO.sub.4 1.5   (0.230Ah) (0.224Ah)     change 120-6 EX HgO 2.5 1.55 Cu D-8Z10 30 ZnO 5.4 N-3 D 10 hrs @ 1.42 hr       0 0 0   ZnSnO.sub.3 3.0 1.55    Zn(OH).sub.2 3.1   23 ma 150 ma 37 39     0 0 0 no discernable shape         KMnO.sub.4 1.5   (0.230Ah) (0.213Ah)         change 121-2 EW HgO 2.5 1.5 Cu D-8Z10 30 ZnO 5.4 N-3 D 10 hrs @ 1.42     hr   0 0 0   ZnSnO.sub.3 2.0 1.45    Zn(OH).sub.2 3.1   23 ma 150 ma 29     39 0 0 0 almost no shape change         KMnO.sub.4 1.5   (0.230Ah)     (0.213Ah) 121-4 EW HgO 2.5 1.5 Cu ZS24- 30   N-3 D 10 hrs @ 1.42 hr   0     0 0 considerable shape change   ZnSnO.sub.3 2.0 1.45  D3  KMnO.sub.4 1.5       23 ma 150 ma 29 39 0 0 0 and holes in anode         ZnSnO.sub.3 7.4     (0.230Ah) (0.213Ah)      structure 121-6 EW HgO 2.5 1.55 Cu S6-D8 30 ZnO     5.4 N-3 D 10 hrs @ 1.49 hr   0 0 0   ZnSnO.sub.3 2.0 1.5    KMnO.sub.4     1.5   23 ma 150 ma 29 39 0 0 0 very little shape change         K.sub.2     SnO.sub.3 1.9   (0.230Ah) (0.224Ah)                 1 0 0 123-5 EW HgO     2.5 1.45 Cu 30K35 30 ZnO 4.3 N-3 D 10 hrs @ 1.27 hr 18 34 0 0 0 a little     shape change   ZnSnO.sub.3 2.0 1.4        23 ma 150 ma      but also low     electrical             (0.230Ah) (0.191Ah)      output 124-3 FB HgO 2.4     1.5 Cu D-8 30 ZnO 5.5 N-3 D 10 hrs @ 1.48 hr   0 0 0   K.sub.2 SnO.sub.3     1.9 1.55    KMnO.sub.4 1.6   23 ma 150 ma 30 34 0 0 0   ZnSnO.sub.3 1.9            (0.230Ah) (0.221Ah) 110-1 EF HgO 2.5 1.55 Cu D-4 30 ZnO 4.2 N-3 D     10 hrs @ 1.51 hr   0 0 0   SnO.sub.2 4.0 1.55    KMnO.sub.4 1.6   23 ma     150 ma 26 30 0 0 0 a little shape change             (0.230Ah) (0.226Ah)     110-2 EG HgO 2.5 1.55 Cu D-4 30 ZnO 4.2 N-3 D 10 hrs @ 1.43 hr   0 0 0     SnO.sub.2 4.0 1.55    KMnO.sub.4 1.6   23 ma 150 ma 26 30 0 0 0 a little     less shape change             (0.230Ah) (0.215Ah)      than 110-1 110-6     EK HgO 2.5 1.55 Cu D-4 30 ZnO 4.2 N-3 D 10 hrs @ 1.44 hrs   0 0 0 very     little shape change   Sn-325 1.0 1.55    KMnO.sub.4 1.6   23 ma 150 ma     26 30 0 0 0             (0.230Ah) (0.216Ah) 110-5 EJ HgO 2.5 1.55 Cu D-4     30 ZnO 4.2 N-3 D 10 hrs @ 1.42 hrs   0 0 0 very little shape change     Sn-325 2.0 1.55    KMnO.sub.4 1.6   23 ma 150 ma 26 30 0 0 0 some tin     flecks visible             (0.230Ah) (0.213Ah)      in separators              0 0 0  98-4 AS HgO 2.5 1.45 Ni D-4 30 ZnO 4.2 N-3 D 10 hrs @     1.48 hrs 20 30 0 0 0 very little shape change   Cu-400 2.0 1.4     KMnO.sub.4 1.6   23 ma 150 ma      in anode width top to     (0.230Ah) (0.223Ah)      bottom but definitely                    change     in thickness top to 100-1 BA HgO 2.5 1.35 Ni D-4 30 ZnO 4.2 N-3 D 10 hrs     @ 1.53 hrs   0 0 0 bottom   Pb-325 2.0 1.4    KMnO.sub.4 1.6   23 ma 150     ma 20 30 0 0 0             (0.230Ah) (0.226Ah)      shape change ˜     98-4 100-2 BB HgO 2.5 1.45 Ni D-4 30 ZnO 4.2 N-3 D 10 hrs @ 1.52 hrs   0     0 0   Pb-325 6.0 1.4    KMnO.sub.4 1.6   23 ma 150 ma 20 30 0 0 0 shape     change ˜     NOTES     .sup.(a) Weight in grams is listed for each of the two anodes in each     cell. Weight is for the dried slurry composition but does not include the     weight of the current collector.     .sup.(b) Current collectors are copper, nickel or electroplated nickel as     indicated. All collectors are made of a fine mesh expanded metal, type     74/0, manufactured by Exmet Corporation, Bridgeport, Connecticut.     .sup.(c) Separator system is as shown in Table 5 and accompanying     description N2 indicates use of nonwoven nylon type 2505K4 with random     fiber orientation and N3 indicates type 2503K4 with unidirectional fiber     orientation. Both types are sold under the trade name PELLON and are     manufactured by Pellon Corporation, Lowell, Massachusetts.     .sup.(d) Initial cathode condition as installed in the cell is indicated     as "C" for fully charged or as "D" as fully discharged.     .sup.(e) Separator #1 is that closest to the anode, #2 is the centrally     located separator and #3 is the separator facing against the cathode. A     number in the separator column indicates that specific number of dendrite     as having penetrated to or thru the particular separator. The letter X     indicates many dendrites and F indicates a few.     .sup.(f) Constant current charging and discharging were employed.     .sup.(g) Unless otherwise noted, shape change notations indicate a     widening and thickening of the expanded anode material increasing in     magnitude from the top toward the bottom. All cells have been run with th     anodes in a vertical position.

Table 9 shows the dendrite and shape change properties of cells thathave been cycled without the initial electrochemical anode oxidationstep. In each case, cells shown in Table 9 can be compared withotherwise identically constructed and tested cells in Tables 6, 7 and 8.

The cells referred to in these tables have all been constructed usingthe double anode-single cathode configuration previously described andshown schematically in FIG. 1. The dimensions of the electrodes,separator system and cell envelope and the cell assembly procedure arethe same as indicated in this earlier description. This includes anominal nickel cathode capacity of 0.250 ampere-hours.

A variety of cell operating conditions have been used. These range from6-hour to 10-hour charge cycles at different constant current levels.Both single and step charging methods have been shown in the examples.The anode current densities calculated for the apparent surface area,using the initial anode dimensions, range from 6.2 ma/cm² to 1.4 ma/cm²during the charging cycle and 9.3 ma/cm² during the discharge cycle.

The specific charge and discharge cycle used for each cell example hasbeen listed in the table. Cycling of the cells has been carried out inthe manner previously described, including the use of the transistorizedcircuitry to cut off the discharge of each cell when its voltage hasreached the 0.9 volt level. In the case of nickel/zinc cells, as used inthese tests, this represents a nearly complete discharge. The totalnumber of charge/discharge cycles given each cell can be found byreferring to the column in each table entitled "cell disassembly atcycle#".

Examination for the presence of dendrites has been made during thedisassembly of the individual cells.

    TABLE 9      COMPARISON TEST CELL EXAMPLES RUN WITHOUT THE INITIAL ELECTROCHEMICAL     ANODE OXIDATION PROCESS OF THE INVENTION    Per-  Cur- Electro-   Per-         Dendrite   Anode  cent  rent lyte   cent  Initial   Dis- Cell     Penetration  Type  Addi-  Col- Type  Major Elec- Sepa- Cath-   charge     Disas- Thru Test (See Major tive in Anode lector (See Per- Electro-     trolyte rator ode Cell Cell Measur- sembled Separators Anode Shape     Change.sup.(g) Cell Table Anode Slurry Weights.sup.(a) Mate- Table cent     lyte Addi- Sys- Con- Charge Discharge ed at at Indicated.sup.(e)     Notations and Special # 1) Additives Mix (grams) rial.sup.(b) 2&3&4) KOH A     dditives tive tem.sup.(c) dition.sup.(d) Cycle.sup.(f) Cycle.sup.(f)     Cycle # Cycle # 1 2 3 Remarks       122-1 I HgO 2.5 1.65 Cu  30K35 30 ZnO 4.3 N-3 C 10 hrs @ 0 hr   1 1 1     at least 1 large dendrite     1.6        23 ma 150 ma 29 31 ? ? ?     probably many small dendrites             (0.230Ah) (0 Ah)      but are     now highly oxidized                    due to shorted cell condition     cell shorted by cycle 15     large shape change, esp. due                    to expansion at bottom     of                    anode 122-2 I HgO 2.5 1.6 Cu D-4 30 ZnO 4.2 N-3 C     10 hrs @ 0 hr   X F 3 large dendrites shorting to     1.65    KMnO.sub.4     1.6   23 ma 150 ma 29 31 X F 1 cathode             (0.230Ah) (0 Ah)     cell shorted by cycle 15     large shape change mostly due                    to expansion at bottom     of                    anode 122-3 I HgO 2.5 1.65 Cu S6 30 ZnO 4.2 N-3 C     10 hrs @ 0 hr   X X X 1 large end many small     1.6    K.sub.2      SnO.sub.3 1.9   23 ma 150 ma 29 31 X ? ?  dendrites shorted to cathode                (0.230Ah) (0 Ah)     cell shorted by cycle 17     large shape change mostly due                    to expansion at bottom     of                    anode 122-4 I HgO 2.5 1.65 Cu S6-D4 30 ZnO 4.2 N-3     C 10 hrs @ 1.43 hr   X X X many small dendrites thru     1.6    KMnO.sub.     4 1.6   23 ma 150 ma 29 31 X X X all separators thru to         K.sub.2     SnO.sub.3 1.9   (0.230Ah) (0.215Ah)      cathode but not yet shorting                     to cathode     extreme shape change incl.                    large expansion at bottom                       of anode 122-5 EW HgO 2.5 1.45 Cu 30K35 30 ZnO 4.3 N-3     C 10 hrs @ 0 hr   X F 2 2 large dendrites shorted to   ZnSnO.sub.3 2.0     1.45        23 ma 150 ma 29 31 X 0 0 cathode             (0.230Ah) (0     Ah)     3/4      cell output by cycle 15                                              ext     reme shape change incl.                    large expansion at bottom                    of anode 122-6 EW HgO 2.5 1.45 Cu D-4 30 ZnO 4.2 N-3 C 10     hrs @ 1.15 hr   X X 1     1 large dendrite shorted   ZnSnO.sub.3 2.0 1.45    KMnO.sub.4 1.6   23     ma 150 ma 29 31 X F ? to cathode             (0.230Ah) (0.173Ah)        3/4 cell output by                    cycle 17         extreme shape change                    including large           expansion at bottom of                    anode     NOTES     .sup. (a) Weight in grams is listed for each of the two anodes in each     cell. Weight is for the dried slurry composition but does not include the     weight of the current collector.     .sup.(b) Current collectors are copper, nickel or electroplated nickel as     indicated. All collectors are made of a fine mesh expanded metal, type     74/0, manufactured by Exmet Corporation, Bridgeport, Connecticut.     .sup.(c) Separator system is as shown in Table 5 and accompanying     description. N2 indicates use of nonwoven nylon type 2505K4 with random     fiber orientation and N3 indicates type 2503K4 with unidirectional fiber     orientations. Both types are sold under the trade name PELLON and are     manufactured by the Pellon Corporation, Lowell, Massachusetts.     .sup.(d) Initial cathode condition as installed in the cell is indicated     as "C" for fully charged or as "D" for fully discharged.     .sup.(e) Separator #1 is that closest to the anode, #2 is the centrally     located separator and #3 is the separator facing against the cathode. A     number in the separator column indicates that specific number of dendrite     as having penetrated to or thru the particular separator. The letter X     indicates many dendrites and F indicates a few.     .sup.(f) Constant current charging and discharging were employed.     .sup.(g) Unless otherwise noted, shape change notations indicate a     widening and thickening of the expanded anode material increasing in     magnitude from the top toward the bottom. All cells have been run with th     anodes in a vertical position.

For simplication and ease in making comparisons between cells, thetables list dendrite penetration through each of the individualseparators. The separator designated (1) is that nearest the anode, (2)is in the middle, and (3) is facing against the cathode. Two sets ofdendrite penetration data are included for each cell because of thedouble anode arrangement. A number appearing in the separator columnindicates that this specific number of dendrites has reached or extendedbeyond the surface of the particular separator. A letter "X" indicatesmany dendrites and the letter "F" indicates a few dendrites. Allexaminations for dendrites were made using a microscope so that even thevery smallest could be detected and recorded. Significant dataconcerning anode shape change has also been included in each table.

A variety of anode current collector materials have been employed inthese tests. These include nickel and copper as well as tin, silver andcopper plated nickel. All of these current collectors were fabricatedfrom a fine mesh expanded metal as explained in more detail in the tablenotes. No significant advantage of one type over another could be seenin these particular tests.

Referring in more detail to Table 6, it will be noted that the varioustest cells listed have been assembled with KOH electrolyte, KOHelectrolyte with zincate present; and, KOH with zincate plus theaddition of an alkaline soluble manganese compound.

Test cell 71-3 employed the basic KOH electrolyte with no zincatepresent while cells 64-1, 64-3, 71-1, 76-1, 80-1, 80-3, 80-5, and 123-1contain KOH electrolyte to which zincate has been added. This lattergroup of cells were found to have, at most, only a few dendrites presentafter the charge/discharge cycling indicated. When present, these wereusually found to have not extended beyond the first separator. On theother hand, cell 71-3, with no zincate present in the initialelectrolyte, showed considerably more dendrite growth and separatorpenetration than the cells containing the zincate. This cell was alsofound to contain badly eroded and "wrinkled" anode surfaces. This anodedistortion problem has been found to occur in all cells that use thespecial electrochemical anode oxidation step of this invention, whenzincate has been omitted from the electrolyte.

It should be noted that cells 64-1 and 64-2 employed charged cathodesand a 10-hour shorting cycle to accomplish the special electrochemicalanode oxidizing step. The more normal procedure has been to short theassembled cells for about 22 hours starting with discharged cathodes.

The remaining test cells in Table 6 use KOH electrolyte solutions inwhich an alkaline soluble manganese compound has been added. As can benoted from an examination of the data, the inclusion of the manganese inthe zincate containing KOH electrolytes has effectively eliminated thedendrite penetration into the separator system. Its use has also beenfound to minimize overall anode shape change effects over those ofsimilar electrolyte formulations that do not contain the manganeseadditive.

Test cells 79-7 and 80-7 employed half the amount of manganese added tothe electrolyte than was used in the majority of the other manganesecontaining cells of Table 6. Cell 80-8 used one-fourth of this amount.It will be noted that these cells also showed no dendrite growth in thenumber of charge/discharge cycles run. This indicates that only a verysmall percentage of the manganese additive is required to limit dendritegrowth at the special anodes of this invention.

Test cell 92-10 shows an example of a cell in which a considerableexcess of manganese was present in the electrolyte. In this case, finelyground potassium permanganate crystals were placed in the bottom of thecell envelope prior to insertion of the electrode separator system. Theidea was to furnish a reservoir that could supply additional solublemanganese to the electrolyte during the operation of the cell. In thisinstance, the electrolyte used (type D-4) was already at its nominalmanganese saturation level with the inclusion of 1.6% potassiumpermanganate in its composition. The excess potassium permanganate addedin powdered form was found to be considerably greater in amount thancould be dissolved in the electrolyte during the entire test period. Atthe conclusion of the test, the electrolyte was found to be still purplein color, indicating that an excess of soluble manganese in the +7valance oxidation state was present. This cell also showed no evidenceof dendrite formation in the separator system.

All other cells in Table 6 that were cycled with the more nominalamounts of manganese containing electrolytes, were found to haveconverted to a reduced manganese form after even a single operatingcycle. This was evident by the fact that the electrolyte was no longerpurple in color and the presence of the manganese was indicated only bybrownish deposits at the anodes, separators and as a precipitate in theelectrolyte. These tests show that effective dendrite protection can beachieved with electrolytes in which the amount of soluble manganese canvary from as little as 0.14% (calculated as manganese) to that of asaturated to super saturated manganese content.

Special mention should be made concerning test cell 71-4. This is theonly cell in Table 6 that showed dendrite penetration into the separatorsystem and also included alkaline soluble manganese in its electrolyte.This cell, however, is the only one using a manganese containingelectrolyte without zincate being present. This cell was included in thetable to illustrate the importance of the use of the manganese incombination with the zincate in the electrolyte solution, and toexemplify the discussion of this phenomenon made earlier in thisdisclosure.

Table 7 shows data obtained with a number of cells operated withelectrolytes containing soluble tin. These additives have includedpotassium and zinc stannate and have been used in electrolytes both withand without zincate and manganese. A wide variation in amounts of thetin additive to the electrolyte have been found useful. As in thepreceding table, these cells have utilized the special electrochemicallyoxidized anodes of this invention.

Cells 118-1 and 118-2 in Table 7 show the effects of using a KOHelectrolyte with the soluble tin additive but without the presence ofzincate. As in the non-zincate example of the preceding table, thesecells showed dendrites along with very irregular anodes withconsiderable shape change and distortion.

Cells 73-3, 111-4 and 123-4 provide examples in which zincate as well asa soluble tin compound has been included in the electrolyte. As can beseen, the use of both the zincate and the tin has greatly reduced anodeshape change effects.

The addition of manganese to these tin/zincate containing electrolyteshas provided further improvement in anode shape change as well as areduction in dendrite growth. Direct comparisons between otherwiseidentical cells, with and without the manganese electrolyte additive,can be made between cells 73-4 and 73-3, 111-2 and 111-4 or 123-4 and123-3.

Cells 115-6 and 116-3 in Table 7 utilized zinc permanganate rather thanpotassium permanganate as the means of incorporating the alkali solublemanganese in these tin containing electrolytes. Zinc permanganate has ahigher solubility in the KOH electrolyte than does the potassiumcounterpart. As a result, a greater manganese content in the electrolytewas present in these cells than in the others shown in this table.

When large amounts of soluble tin have been incorporated in theelectrolyte, such as in the case of cells 116-4, 116-5, 116-6 and 118-2in Table 7, very fine crystalline deposits can be seen in the separatorsystem under microscopic examination. These fine crystalline depositsare mostly concentrated in the separator nearest the anode. Visuallythey do not appear to be interconnected in any way and no noticeableeffect on cell operating conditions has been noted. The presence oflarger tin flakes as intersticial anode deposits was also noted in thesecells. As described earlier in this disclosure, this is a commoncondition even when much lower tin concentrations have been used in theelectrolyte.

Overall, the tests of Table 7 have shown that the inclusion of tin inthe cell electrolyte will provide a reduction in anode shape change. Theaddition of manganese along with the tin provides still greater shapechange reduction. The manganese, however, is still found to be primarilyresponsible for dendrite prevention. Zincate must be present in eithercase to prevent serious anode distortion.

Table 8 shows a number of test cell configurations in which selectedinorganic additives have been included in the initial anodeconstruction. In this way, the additives were present in the anodestructure during their special electrochemical oxidation process.

Anode additives that have been found to be particularly useful forreducing shape change, include zinc stannate, stannic oxide and tinmetal. Potassium stannate can also be included in small amounts. Theaddition of finely divided copper or lead metal powders to the anode mixwere also found to provide an improvement in shape change. Examples ofcells using anodes of each of these types are included in Table 8.

All but one cell in Table 8 use zincate containing electrolyte with asoluble manganese additive. In addition, a number of these cells alsoinclude soluble tin compounds included in the electrolyte. Cell 123-5contains a KOH electrolyte with zincate only. This is the only cell inthe table that showed dendrite growth into the separators.

An overall analysis of the cells in Table 8 shows that selectedcompounds, especially those containing tin, are helpful in minimizinganode shape change. These anodes when used in combination with zincatecontaining electrolytes, plus soluble manganese can provide dendritefree anodes with minimal shape change even after repeated cell cycling.Soluble tin may also be included in the cell electrolyte to augmentanode additives.

Table 9 includes a total of six cells that have been run for 31charge/discharge cycles without the use of the preliminaryelectrochemical anode oxidation step of this invention. These can becompared with similar cells included in preceding Tables 6, 7 and 8 thathave used the special pre-oxidation anode processing. As examples, cells122-1 and 122-2 in Table 9 can be compared with 80-1 and 80-2 in Table6, 122-3 and 122-4 can be compared with 123-3 and 123-4 in Table 7 and122-6 with 121-6 in Table 8.

As can be seen from an examination of Table 9, zincate containing KOHelectrolytes were employed in each instance; however, examples areincluded in which soluble manganese, tin or a combination of the twohave also been incorporated in the electrolyte. The cells were assembledwith charged nickel cathodes since the anodes were already in a reducedform at the start of the cycling process. As in the tests of Tables 6, 7and 8, only non-woven nylon separators were used between anodes andcathodes. No microporous membranes were included.

Overall operating results for these cells were typical and extremelypoor. While all six cells were providing full output at their thirddischarge cycle, when next checked at 15 cycles, cells 122-1 and 122-2were found to be dead and 122-5 had about 3/4 normal output. By cycle17, cell 122-3 was found to be dead and 122-6 was now providing about3/4 output. Cell 122-5 had regained almost full output . . . undoubtedlydue to the oxidation of some previously contacting dendrites. Only cell122-4 showed full output by the end of the 31 cycle test, although ithad given less than full output on a couple of earlier discharge cycles.

Cells 122-1, 122-2 and 122-3 that had obviously shorted early in thecycling, and had remained in this condition, were found on disassemblyto have one or two large dendrites. These had penetrated from the anode,grown through all three separators and were found to be firmly makingcontact against the nickel cathode. Because of the early cell shorting,only a moderate number of dentrites had formed in these cells. On theother hand, cell 122-4 that had retained a more or less normal outputthrough the cycling period, was found to contain extremely large numbersof dendrites. These had penetrated all three separators on both sides ofthe cathode, had even grown around the edges of the cathode and hadbridged across the bottom of the separators from one side to the other.Cells 122-5 and 122-6 also had extremely large numbers of dendrites. Allsix cells had abnormally large amounts of shape change at the anodes.Cells 122-4, 122-5 and 122-6 had extremely severe shape change.

These test results have essentially repeated many earlier attempts touse conventional zinc anodes without the special pre-oxidation step.They have included anodes made from zinc dust, chemical zinc oxide andcombinations of the two.

The negative results of Table 9 exemplify the extreme improvement thatcan be achieved in the reduction of dendrite growth and shape changethrough the use of anodes that have been converted from metallic zinc toa highly electrochemically oxidized state prior to the start of normalcell cycling.

Table 9 also points out that the use of soluble manganese and/or tinincluded in the electrolyte, or tin included in the anode, is of noparticular benefit in reducing dendrite and shape change effects unlessthe specially oxidized anodes of this invention are involved.

The preceding disclosure and examples are intended to illustrate and notto limit this invention. Variations and modifications can be madetherein without departing from its intended scope.

What is claimed is:
 1. A method of making an anode for a battery havinga zinc containing anode which comprises forming a slurry of finelydivided zinc metal particles, applying a coating of the slurry to ametal substrate, drying the slurry, immersing the coated substrate in anaqueous alkaline electrolyte solution, positioning a cathode in theelectrolyte, electrically connecting the anode with the cathode untilthe cathode is substantially completely discharged as evidenced by theevolution of hydrogen gas therefrom and maintaining the electricalconnection for a period of time sufficient to oxidize in situsubstantially all of the zinc metal in the coating to zinc oxide asevidenced by substantial cessation of hydrogen evolution at the cathode.2. The method of claim 1 wherein the zinc metal particle slurry includesa mercury compound.
 3. The method of claim 1 wherein the metal substrateis perforated and the metal is selected from the group consisting ofnickel, copper, tin, silver, gold plated nickel, and copper or tinplated iron or steel.
 4. The method of claim 1 wherein the electrolyteis an alkali metal hydroxide containing an amount of zincate.
 5. Themethod of claim 4 wherein the electrolyte is potassium hydroxide.
 6. Themethod of claim 4 wherein the electrolyte includes an additive selectedfrom the group consisting of manganates and permanganates.
 7. The methodof claim 1 wherein the oxidation step is continued until at least about95% of the zinc metal is converted to the oxide.
 8. The method of claim1 wherein the electrolyte includes an additive selected from the groupconsisting of manganates and permanganates.
 9. The method of claim 1wherein the alkaline electrolyte solution contains an alkali solubleinorganic compound additive of an alkaline soluble manganese compound.10. The method of claim 1 wherein the electrolyte is an alkali metalhydroxide containing an amount of a stannate.
 11. The method of claim 1wherein the electrolyte is an alkali metal hydroxide containing anamount of a zincate and a stannate.
 12. The method of claim 1 whereinthe electrolyte is an alkali metal hydroxide containing an amount of azincate, a stannate and a permanganate or manganate.